Pharmacological and direct electrical neuromodulation techniques have been employed in various interventional settings to address challenges such as prolonged orthopaedic pain, epilepsy, and hypertension. Pharmacological manipulations of the neural system may be targeted to certain specific cell types, and may have relatively significant physiologic impacts, but they typically act on a time scale of minutes, whereas neurons physiologically act on a time scale of milliseconds. Electrical stimulation techniques, on the other hand, may be more precise from an interventional time scale perspective, but they generally are not cell type specific and may therefore involve significant clinical downsides. A new neurointerventional field termed “Optogenetics” is being developed which involves the use of light-sensitive proteins, configurations for delivering related genes in a very specific way to targeted cells, and targeted illumination techniques to produce interventional tools with both low latency from a time scale perspective, and also high specificity from a cell type perspective.
For example, optogenetic technologies and techniques recently have been utilized in laboratory settings to change the membrane voltage potentials of excitable cells, such as neurons, and to study the behavior of such neurons before and after exposure to light of various wavelengths. In neurons, membrane depolarization leads to the activation of transient electrical signals (also called action potentials or “spikes”), which are the basis of neuronal communication. Conversely, membrane hyperpolarization leads to the inhibition of such signals. By exogenously expressing light-activated proteins that change the membrane potential in neurons, light can be utilized as a triggering means to induce inhibition or excitation.
One approach is to utilize naturally-occurring genes that encode light-sensitive proteins, such as the so-called “opsins”. These light-sensitive transmembrane proteins may be covalently bonded to chromophore retinal, which upon absorption of light, isomerizes to activate the protein. Notably, retinal compounds are found in most vertebrate cells in sufficient quantities, thus eliminating the need to administer exogenous molecules for this purpose. The first genetically encoded system for optical control in mammalian neurons using light-sensitive signaling proteins was established in Drosophila melanogaster, a fruit fly species, and neurons expressing such proteins were shown to respond to light exposure with waves of depolarization and spiking. More recently it has been discovered that opsins from microorganisms which combine the light-sensitive domain with an ion pump or ion channel in the same protein may also modulate neuronal signaling to facilitate faster control in a single, easily-expressed, protein. In 2002, it was discovered that a protein that causes green algae (Chlamydomonas reinhardtii) to move toward areas of light exposure is a light-sensitive channel; exposure to light of a particular wavelength (maximum results at blue light spectrum i.e., about 480 nm) for the opsin ChR2, also known as “channelrhodopsin”) causes the membrane channel to open, allowing positive ions, such as sodium ions, to flood into the cell, much like the influx of ions that cause nerve cells to fire. Various other excitatory opsins, such as Volvox Channelrhodopsin (“VChR1”), Step Function Opsins (or “SFO”; ChR2 variants which can produce prolonged, stable, excitable states with blue-wavelength light exposure, and be reversed with exposure to green-wavelength light, i.e., about 590 nm), or red-shifted optical excitation variants, such as “C1V1”, have been described by Karl Deisseroth and others, such as at the opsin sequence information site hosted at the URL: http://www.stanford.edu/group/dlab/optogenetics/sequence_info.html, the content of which is incorporated by reference herein in its entirety. Examples of opsins are described in U.S. patent application Ser. Nos. 11/459,638, 12/988,567, 12/522,520, and Ser. No. 13/577,565, and in Yizhar et al. 2011, Neuron 71:9-34 and Zhang et al. 2011, Cell 147:1446-1457, all of which are incorporated by reference herein in their entirety.
While excitation is desirable in some clinical scenarios, such as to provide a perception of a sensory nerve stimulation equivalent, relatively high-levels of excitation may also be utilized to provide the functional equivalent of inhibition in an “overdrive” or “hyperstimulation” configuration. For example, a hyperstimulation configuration has been utilized with capsaicin, the active component of chili peppers, to essentially overdrive associated pain receptors in a manner that prevents pain receptors from otherwise delivering pain signals to the brain (i.e., in an analgesic indication). An example of clinical use of hyperstimulation is the Brindley anterior sacral nerve root stimulator for electrical stimulation of bladder emptying (Brindley et al. Paraplegia 1982 20:365-381; Brindley et al. Journal of Neurology, Neurosurgery, and Psychiatry 1986 49:1104-1114; Brindley Paraplegia 1994 32:795-805; van der Aa et al. Archives of Physiology and Biochemistry 1999 107:248-256; Nosseir et al. Neurourology and Urodynamics 2007 26:228-233; Martens et al. Neurourology and Urodynamics 2011 30:551-555). In a parallel manner, hyperstimulation or overdriving of excitation with an excitatory opsin configuration may provide inhibitory functionality. It may also be referred to as a hyperstimulation block when used to produce a depolarization block.
Other opsin configurations have been found to directly inhibit signal transmission without hyperstimulation or overdriving. For example, light stimulation of halorhodopsin (“NpHR”), a chloride ion pump, hyperpolarizes neurons and directly inhibits spikes in response to yellow-wavelength (˜589 nm) light irradiation. Other more recent variants (such as those termed “eNpHR2.0” and “eNpHR3.0”) exhibit improved membrane targeting and photocurrents in mammalian cells. Light driven proton pumps such as archaerhodopsin-3 (“Arch”) and “eARCH”, and ArchT, Leptosphaeria maculans fungal opsins (“Mac”), enhanced bacteriorhodopsin (“eBR”), and Guillardia theta rhodopsin-3 (“GtR3”) may also be utilized to hyperpolarize neurons and block signaling. Direct hyperpolarization is a specific and physiological intervention that mimics normal neuronal inhibition. Suitable inhibitory opsins are also described in the aforementioned incorporated by reference resources.
Further, a ChR2 variant known as a Stabilized Step Function Opsin (or “SSFO”) provides light-activated ion channel functionality that can inhibit neural activity by depolarization block at the level of the axon. This occurs when the depolarization results in a depolarized membrane potential such that sodium channels are inactivated and no action potential of spikes can be generated.
C1V1-T refers to C1V1 (E122T) or C1V1 (E162T). C1V1-TT refers to C1V1 (E122T/E162T).
The term light-sensitive protein, as used herein, refers to all the aforementioned types of ion channels and ion transporters/pumps in the context of modulating a membrane potential.
With a variety of opsins available for optogenetic experimentation in the laboratory, there is a need to bring such technologies to the stage of medical intervention, which requires not only a suitable selection of opsin-based tools for excitation and/or inhibition, but also a means for delivering the genetic material to the subject patient and a means for controllably illuminating the subject tissue within the patient to utilize the light-driven capabilities. There is a need for practical configurations and techniques for utilizing optogenetic technologies in the clinical setting to address various clinical challenges of modern medicine with specificity and temporal control precision.